Radial turbines and compressors, such as those used in turbochargers, typically include a rotor, or wheel, that is rotatably mounted in a housing and that defines blades extending radially outward in proximity to an inner surface of the housing. The housing defines an inlet for receiving air or other gas, and an outlet through which the gas is circulated. In the case of a turbine, the rotor is a turbine wheel that is rotatably mounted in a turbine wheel housing. Gas, such as exhaust gas from an internal combustion engine, flows into the housing through the inlet, which extends circumferentially around the wheel, and exits in a generally axial direction. As the gas passes through the housing, the turbine wheel is rotated. In a typical turbocharger, the turbine wheel is connected by a shaft to a compressor wheel, i.e., a rotor, that is rotatably mounted in a compressor wheel housing. The compressor wheel housing also defines an inlet and outlet, and the compressor wheel includes radial blades that deliver air through the compressor wheel housing. In particular, the compressor wheel draws air axially inward through the inlet and delivers the air radially outward through a diffuser that extends circumferentially around the compressor wheel.
The blades of the rotors of turbines and compressors typically have edges that are positioned in close proximity to the housing and other relatively stationary components. For example, the turbines and compressors of modern turbochargers can include stators at the inlet and/or outlet to control the flow of gas through the device. In a turbine, the stators can be vanes arranged circumferentially at the inlet to define a stationary or an adjustable nozzle. The nozzle can be selectively opened and closed to control the flow of the gas through the turbine. In a compressor, the stators can be vanes that are arranged circumferentially at the outlet to define a variable diffuser that controls the flow of the air through the compressor. Due to the close proximity of the rotors and stators, the high rotational speeds of the rotors, and the high operating pressures, the blades of the rotors are subject to unsteady aerodynamic excitation forces that induce alternating strains and stresses in the blades. Such unsteady, or cyclic, excitation forces can similarly result from other stationary or adjustable components such as inlet guide vanes or a curved inlet manifold that supplies the gas to the inlet at pressures that vary across the area of the inlet. For example, inlet guide vanes are often provided in the inlet of a compressor to direct the flow of air therethrough. Thus, the blades are cyclically stressed at frequencies associated with the rotational speed of the rotor and the number and location of the vanes or other stationary components. Such cyclic stress can result in fatigue and failure of the rotors.
A forced response analysis can be conducted during the design of a rotary device such as a turbine or compressor to determine the cyclic stresses and strains on the rotor due to any unsteady aerodynamic excitation forces that occur at the rotor's resonant frequencies. The unsteady aerodynamic mechanical response of the rotor can be first analyzed, e.g., by conducting a computational fluid dynamics (CFD) analysis to determine the unsteady aerodynamic excitation forces, and conducting a 3-dimensional finite element method (FEM) analysis to determine the natural resonant frequencies of the rotor. Typically, the geometric configuration of the rotor or other components of the device is adjusted or modified as is practical to reduce the stresses and strains of the rotor that result from the unsteady aerodynamic excitation forces, e.g., by adjusting the configuration of the rotor or other devices such that the resonant frequencies occur outside the operating range of the rotor. The normal operating range of the device may be such that the rotor is not significantly stressed when subjected to cyclic aerodynamic forces that correspond to the lowest of the resonant frequencies of the rotor due to the low speed and pressure associated with that speed of operation. However, it is often impossible or impractical to adjust the higher resonant frequencies out of the operating speed range of the turbocharger. Thus, for example, the rotor may be subjected during some times of operation to a cyclic aerodynamic excitation force having a frequency that is equal to the second mode or higher modes of the resonant vibratory frequency of the rotor. Accordingly, the design analysis can include determining the strains and stresses that occur in the rotor at such frequencies and verifying that the expected life of the rotor meets a minimum design criteria. In some cases, however, the rotor may be subjected to alternating strains that reduce the expected life of the rotor below a minimum design criteria.
Thus, there exists a need for improved rotors for rotary devices such as turbines and compressors that are used in turbochargers, and for a method of manufacturing such devices. Preferably, the devices should be subjected to reduced strains and stresses, thereby extending the operating lives of the devices, despite cyclic aerodynamic excitation forces, which can occur throughout the operating range of the device, including at one or more of the vibratory modes of the rotor of the device.